Many prior wave energy converters employ additional wave-activated mediums such as air (air-turbine) or hydraulic (hydraulic-motor) to produce mechanical motion to drive an electric generator rather than allowing the wave-induced fluid flow to directly turn a water-turbine generator. This incurs additional energy conversion losses before electric power is generated to the grid. Air turbines positioned above the water surface must consider corrosive effects due to mechanical parts being exposed to a salt-laden air/water boundary and are also noisy. Air, as a compressible intermediary mechanism, being less dense than water, also results in more complex mechanical designs needed to harvest the wave energy. For the same flow rate as water, air as an active medium requires more surface area to generate the same amount of power as water.
Prior wave overtopping devices generate hydroelectric power by creating a higher surface elevation (head) relative to the still water level. These devices have the disadvantage of not only being in direct contact with the surface waves, but also require a large basin to hold the water. In addition, the head must be maintained or the turbine can run dry. Near-shore surface wave energy converters also have the undesirable consequence of being unsightly when viewed from the shoreline, especially in locations where coastal real-estate is at a premium.
The following are examples of other wave conversion devices for generating electrical energy. In U.S. Pat. No. 4,371,788, and U.S. Pat. No. 4,170,728, the disclosed systems extract wave energy by enabling water particles to move a sail beneath the water. U.S. Pat. No. 4,279,124 disclosed the use of propellers mounted on a submerged support, which are turned by waves to extract energy. A device called the “Bristol Cylinder” developed by Dr. D. Evans employed a large submerged cylindrical concrete mass floating beneath the surface, which can be made to move in a circular fashion when following the wave induced water-particle orbital motion, and hydraulic rams are used to pump high pressure oil to turn an electric generator. For a detailed description, see Hagerman, G., “Wave Energy Resource and Economic Assessment for the State of Hawaii.” prepared by SEASUN Power Systems for the Department of Business, Economic Development and Tourism, Final Report (1992). These prior devices have the disadvantages of using intermediary mechanisms for the incoming wave energy, resulting in more complex mechanical designs needed for wave energy conversion.
A wave energy conversion system, described in “About the Development of Wave Energy Breakwaters”, by Graw, K., published in Lacer No. 1, Leipzig Annual Civil Engineering Report, Universitat Leipzig (1996), employed wave-activated pulsating flow beneath a divider plate, to capture wave energy by driving a low-head hydro-turbine also beneath the divider plate. However, this type of device utilized only wave-induced flow below the divider plate and did not make use of the significant wave-induced flow above and through a surface. The orientation of its hydro-turbine for power generation was also more dependent on the incident wave direction.
Another type of device called the “WaveMaster” wave energy converter, developed by Ocean WaveMaster Ltd., of Manchester, U.K., employed a submerged surface to capture wave energy through multiple one-way valves creating zones of high and low pressure water that flows through turbines within the structure. However, the WaveMaster converted wave energy only across the structure and not through or beneath the surface. The one-way valves used result in power being generated on only one-half of the wave cycle. Debris that may accumulate due to flow through the downward check-valves can also be an operation or maintenance concern.
An example of a device for converting water waves into usable energy by a providing a water turbine-generator centrally located in an aperture of a submerged horizontal surface to generate electric power is described in U.S. patent application Ser. No. 12/348,518 filed on Jan. 5, 2009, of which the inventor herein was a co-inventor, and is incorporated by reference herein.